Note Q - Quarterly Financial Data (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues	$ 280,805	$ 288,951	$ 288,949	$ 273,527	$ 276,426	$ 264,796	$ 263,860	$ 245,052	$ 1,132,232	$ 1,050,134	$ 949,303
Operating income	19,038	21,497	18,734	15,962	16,610	18,528	17,574	16,650	75,231	69,362	64,685
Net income	$ 10,750	$ 11,545	$ 9,174	$ 7,900	$ 8,763	$ 11,553	$ 9,998	$ 9,716	$ 39,369	$ 40,030	$ 39,330
Earnings per Share:
Basic (in dollars per share)	$ 0.56	$ 0.60	$ 0.47	$ 0.41	$ 0.45	$ 0.59	$ 0.51	$ 0.49	$ 2.04	$ 2.04	$ 1.99
Diluted (in dollars per share)	$ 0.55	$ 0.59	$ 0.47	$ 0.40	$ 0.44	$ 0.59	$ 0.50	$ 0.48	$ 2	$ 2	$ 1.95
Weighted-average common shares outstanding (in thousands)
Basic weighted-average shares outstanding (in shares)	19,102	19,316	19,475	19,450	19,409	19,450	19,795	19,804	19,335	19,608	19,755
Diluted (in shares)	19,373	19,556	19,706	19,838	19,744	19,713	20,082	20,277	19,663	19,997	20,186